1. Field of the Invention
The invention relates to communication systems where transmitter and receiver are time locked. In particular, the invention describes how the receiver can recover from clock slips.
2. Description of the Related Art
Advances in wireless technology and VLSI has allowed mobile radios and other portable devices to be produced with lower cost, size, and power consumption. As a result, the use of radio communication in consumer applications is becoming more widespread. In addition, it is expected that wireless technology will move beyond the present focus on voice communication (e.g., handheld radios) to include other types of communication. For example, radio communication will be used to connect master devices (e.g., laptop computer) to their respective peripheral devices (e.g., printer).
The aforementioned radio communication will require an unlicensed frequency band with sufficient capacity to accommodate high data rate transmissions. A suitable band is the ISM (Industrial, Scientific and Medical) band at 2.45 GHz, which is globally available. The ISM band provides 83.5 MHz of radio spectrum to be shared amongst all radio networks wishing to use this band.
In order to allow different radio networks to share the radio spectrum without having to coordinate with one another, some form of signal spreading is usually applied. Signal spreading involves scattering the transmitted data across the entire range of available frequencies. Each channel is assigned a unique spreading code which is used to spread the data. The data is spread in such a way that only the same spreading code may be used to recover the data. Spreading can be done either at the symbol level by applying direct-sequence (DS) spread spectrum, or at the channel level by applying frequency hopping (FH) spread spectrum. The latter is more attractive for the radio applications mentioned above, since frequency hopping can more readily incorporate the use of cost-effective radios.
A wireless system called Bluetooth™ has been developed that takes advantage of the 2.45 GHz band. Bluetooth™ provides pervasive wireless connectivity, especially between mobile and portable devices such as mobile phones, laptop computers, PDA, and other nomadic devices as well as fixed devices. The Bluetooth™ system uses frequency hopping, which enables the construction of low-power, low-cost radios with small footprints. Both data and voice communication are supported. The latter is optimized by applying fast frequency hopping in combination with robust voice coding. Frequency hopping is done at a nominal rate of 1600 hops/s through the entire 2.45 GHz band. The dwell time for one hop is 625 μs, which corresponds to the nominal time for one transmission time slot.
Devices based on the Bluetooth™ system can create so-called piconets. A piconet is basically an ad hoc network of two or more Bluetooth™ devices connected together. Each piconet includes a single master device and one or more slave devices. The master device controls all communication in the piconet. Slave devices can only communicate in response to a master device. The master device can communicate with any slave device. Communication is carried over a frequency hopped channel in the 2.45 GHz band mentioned above. All devices in the piconet have to use the same hopping sequence. The hopping sequence is determined by the identity of the master device.
Each Bluetooth™ device has a free-running system clock. The system clock of a master device determines the phase in the hopping sequence used in the piconet. Since the free-running clocks of the master and the slaves are not coordinated, mutual drift will occur. The slave devices then have to align their clocks with the clock in the master device to be able to communicate. The clock alignment keeps the slave devices in phase synchronization with the master device. Clock alignment can be accomplished by adding a time offset to the clocks in the slave devices. For more details, see “The Bluetooth radio system,” J. C. Haartsen, IEEE Personal Communications Magazine, Vol. 7, No. 1, February 2000, pp. 28-36.
The hop sequences used in the Bluetooth™ system are generated through a hop selection mechanism. An example of a hop selection mechanism is described in U.S. patent application Ser. No. 08/950,068, entitled “Method and apparatus for the generation of frequency hopping sequences,” filed on Oct. 24, 1997. See also U.S. patent application Ser. No. 08/932,911, entitled “FH piconets in an uncoordinated wireless multi-user system,” filed on Sep. 18, 1997. Both of these patent applications are hereby incorporated by reference. The patent applications describe a hop sequence selection mechanism wherein the hop carriers are generated “on the fly” because the mechanism has no inherent memory. Instead, address and clock information are used to instantaneously determine the hop sequence and phase and, therefore, the desired hop carrier. The advantages of such a selection scheme are numerous. For example, the selection scheme allows a slave device to jump from one piconet to another by simply selecting a different master address and clock combination.
The frequency hopping sequences used in Bluetooth™ are based on the 28 LSB bits in the master address. For purposes of this description, these 28 bits will be referred to as the master identity. As a result, there are 228 (or 268,435,456) different hop sequences available. The length of each sequence is determined by the master clock, which counts from 0 to 227-1 at a rate of 1600 increments per second. The counting wraps around after about 23.3 hours. The number of possible sequences and the size of each sequence make it impossible to store the sequences and process them off-line. Instead, a sequence selection mechanism such as described in the above referenced patent applications is used.
For proper operation of the Bluetooth™ system, the master and the slave have to remain in frequency hop synchrony. The frequency hopping is driven by the system clock of the master device. At each packet reception, the slave adjusts its clock offset such that the input to the slave's hop selection mechanism is aligned with the input to the master's hop selection mechanism. To maintain synchronization, the slaves use a special synchronization word called the access code in the Bluetooth™ packets. The slaves obtain the access code by scanning the packet they receive from the master.
For the above approach to work properly, the slave has to receive packets from the master on a regular basis. If the time elapsed since the last packet reception becomes too long, there is a risk the access code transmitted by the master will fall outside the scan window of the slave. The slave can widen its scan window to try and catch the access code again (at the expense of power consumption). Nevertheless, an error in the clock alignment will occur if: (1) the misalignment is larger than one time slot, and (2) the considered time slots happen to use the same carrier frequency. The latter scenario can happen when a hopping sequence results in the same carrier frequency for two consecutive or nearby time slots. In that case, the receiver may obtain the right carrier frequency, but the wrong time slot and hence, the wrong clock synchronization. Because correct clock alignment is required to decode the whitening and, in some cases, the encryption performed in Bluetooth™ systems, packets cannot be decoded correctly (if the clocks are not aligned).
Accordingly, it is desirable to provide a more robust way to recover clock alignment in a wireless communication system. Specifically, it is desirable to provide a more robust method and system of recovering clock alignment in a Bluetooth™ system.